SUMMARY: Half of the global human population harbors the gastric bacterial pathogen Helicobacter pylori. Despite a vigorous mucosal immune response, H. pylori survives in its ecological niche. The failure of the host response to the pathogen is the key issue: the persistence of the bacterium leads to gastric inflammation. This chronic gastritis can progress through a histologic cascade that can ultimately result in gastric carcinoma in 1-3% of all those infected. Our laboratory has directly implicated gastric macrophages as major orchestrators of the mucosal innate immune response that also mediate development of adaptive immunity. Efforts are needed to understand more precisely the interaction of the bacterium and the host macrophage response. Our project seeks to further elucidate a new mechanism of gastric inflammation induced by H. pylori. Our novel data show that cystathionine ?-lyase (CTH), an enzyme involved in the mammalian reverse transsulfuration pathway, is induced in cultured macrophages and in gastric macrophages of mice and humans infected with H. pylori. Our data indicate that compared to wild-type animals, mice with deletion of the Cth gene exhibit reduced gastritis, and less macrophage and T cell activation, associated with increased colonization by H. pylori. These findings suggest that CTH is an unrecognized mediator of inflammation. Moreover, we have found that increased CTH activity in infected macrophages regulates polyamine concentrations through the depletion of decarboxylated S- adenosylmethionine (dcSAM), which is required for the conversion of putrescine to spermidine and of spermidine to spermine. Further, we show that decreased putrescine and increased spermidine levels are observed in the gastric tissues of Cth-deficient mice. Importantly, we have reported that both putrescine and spermidine play a critical role in H. pylori gastritis. In this context, we hypothesize that CTH induction in macrophages contributes to innate immune cell activation in response to H. pylori, dysregulating immunometabolism and thus increasing inflammatory response and pathogenesis. This new concept for H. pylori interaction with macrophages will be studied in two Specific Aims. 1) To determine the role of H. pylori-induced CTH in the immunopathogenesis of gastritis; for this, we will determine colonization, gastritis, mucosal immune response, and the metabolomic signature of gastric macrophages and T cells in wild-type and Cth-deficient mice. 2) To determine if dysregulation of polyamine metabolism by CTH causes gastritis through effects on the polyamines putrescine and/or spermidine. Thus, we will A) analyze methionine flux in the reverse transsulfuration pathway and in polyamine metabolism in vivo in WT and Cth?/? mice; then, we will determine whether changes in response to H. pylori in Cth-deficient or WT mice are modulated by i) a putrescine or spermidine supplementation, or ii) an inhibitor of adenosylmethionine decarboxylase 1, the enzyme that synthesizes dcSAM. These in vivo studies and discovery metabolomic findings are expected to provide new insights that can be translated to future investigations in human populations with H. pylori infection and to other bacterial infections of the gastrointestinal tract.